1. Field of the Invention
The present invention generally relates to computer equipment having a cooling fan to cool an inlet air and/or an outlet air. In particular, the present invention relates to a fan control apparatus and a fan control method for controlling the revolving speed of a cooling fan depending on an internal temperature of computer equipment.
2. Description of Related Art
Generally, computer equipment has its casing equipped with a cooling fan to control a temperature increase due to the heating of devices installed therein. For example, it is possible for the computer equipment to employ the generally-known methods for controlling the operation of a cooling fan as follows.    (1) A temperature sensor is attached to a device inside the computer equipment so as to monitor its temperature, and therefore the computer equipment controls the revolving speed of a cooling fan based on the temperature.    (2) The computer equipment monitors the temperature of an inlet air flowing into a device, and therefore the computer equipment controls the revolving speed of a cooling fan based on the inlet temperature.    (3) The computer equipment monitors a current or power applied to a device while monitoring an inlet temperature so as to estimate the temperature status of a device, and therefore the computer equipment controls the revolving speed of a cooling fan based on the inlet temperature and the current applied to a device.
Conventionally, computer systems are equipped with cooling devices, which are disclosed in documents, namely patent literatures (PLT) or patent documents. For example, PLT 1 discloses a cooling device for a server having a processor and a fan. The temperature of a processor after a predetermined time elapses is estimated based on the current temperature and the operating ratio of a process as well as the revolving speed of a fan and the inlet temperature of a server, and therefore the cooling fan is controlled to operate at target revolving speed when the estimated temperature exceeds a predetermined value. PLT 2 discloses a dynamic revolution control device for a cooling fan installed in a computer having first and second devices. A table is provided to store the relationship between the temperature of a computer, the temperature of a first device, the temperature of a second device, and the revolving speed of a cooling fan, and therefore the revolving speed of a cooling fan is determined with reference to the table. PLT 3 discloses a computer system equipped with a battery-mounted system board and a cooling fan. The computer system changes the operating state of a cooling fan depending on whether or not a battery is being charged, and therefore the computer system prevents a temperature increase inside the casing due to heating of a battery being charged. PLT 4 discloses a cooling method for an electronic device. The electronic device controls the cooling degree of a cooling member depending on the attribute of an extension card attached thereto.
The foregoing methods (1) to (3) suffer from the following drawbacks. It is likely that the method (1) cannot optimally control a cooling fan with respect to a device unable to accept a temperature sensor attached thereto. The method (2) may not actually monitor the temperature status of a device; hence, in the worst case scenario (where a device operates with the maximum power), it may operate a cooling fan at extremely high speed. The method (3) may not appropriately control the temperature of each device with respect to an add-in card including multiple devices having different temperature statuses. The foregoing technologies (e.g. PLT 1, PLT 2, and PLT 4) suffer from those drawbacks. PLT 3 is limited to a battery as a cooling object; hence, it is difficult to apply the technology of PLT 3 to other types of devices.